The present invention is directed to an electrical transformer, especially an ignition coil for use particularly in an automotive internal combustion engine, and including generally a magnetic circuit and two windings, one primary and the other secondary.
In certain types of coils, the magnetic circuit includes a central magnetic core member surrounded coaxially by two plastic spools, one of which carries a primary winding and the other a secondary winding, and at least one magnetic current return circuit member located opposite the central magnetic core member.
French patent application 85-11458 discloses an ignition coil of the type described above in which the magnetic circuit and the primary and secondary spools are inserted into a plastic casing prior to pouring a synthetic resin into the casing to stabilize and insulate electrically the elements in the casing.
Ignition coils of this type, although pleasing in appearance, do not always meet the technical requirements demanded of them. When operating under full power, heat tends to cause the magnetic circuit in such coils to change shape and expand, which may produce cracks in the synthetic resin that holds it, thus compromising in spots the electrical insulation provided by the synthetic resin. This leads to the formation of current leaks from the high-voltage secondary winding toward the low-voltage primary winding or to the various metal parts of the ignition coil, e.g. the magnetic circuit.
French patent application 86-10664 would remove this drawback by placing a compressible sleeve between the synthetic resin and all or part of the magnetic circuit of the ignition coil. However, this arrangement sometimes proves insufficient when the magnetic circuit undergoes serious deformation and expansion.
One way of eliminating the problem while retaining the principle of encasing the magnetic circuit within a housing would be to use a synthetic resin having good dielectric properties and an expansion coefficient compatible with that of the magnetic circuit, thereby compensating for the deformation and expansion in the magnetic circuit while at the same time thoroughly insulating the ignition coil electrically. Unfortunately, no resin possessing these two characteristics is currently available at a price that would permit it to be used in competitively priced mass-production coils.